What plant hormone is primarily responsible for promoting fruit ripening?

The hormone primarily responsible for promoting fruit ripening is ethylene. Ethylene is a gaseous plant hormone that plays a critical role in the ripening process of many fruits. It triggers various biochemical pathways that lead to the softening of fruit, the change in color, and the development of characteristic flavors and aromas, all of which are essential for attracting animals that help in seed dispersal.

When fruits begin to ripen, they produce ethylene, which can further stimulate ripening in nearby fruits, demonstrating a form of communication among plants. This is particularly evident in situations where fruits like bananas and apples can influence each other's ripening process when stored together.

In contrast, auxins are primarily involved in cell elongation and growth, cytokinins promote cell division and growth, and gibberellins are associated with stem elongation, seed germination, and other growth processes but do not play a direct role in the ripening of fruits. Thus, ethylene stands out as the essential hormone responsible for driving the ripening process, making it the correct answer.